1. Field of the Invention
The present invention relates to an optical recording apparatus for recording information on an optical recording medium in which a recording surface is covered with a transparent layer.
2. Description of the Related Art
As one of techniques for properly recording marks indicative of information on a recordable optical disk, Strategy recording is known. A concrete example of the Strategy recording is shown in FIGS. 1, 2 and 3.
FIG. 1 is a block diagram showing a schematic construction of a conventional optical recording apparatus using the Strategy recording, FIG. 2 is a timing chart showing the characteristics of four-step controlling of LD power, in which the optical power of a laser beam issued from a laser diode (LD) 85 shown in FIG. 1 and time are assigned on the vertical and lateral axes respectively, and FIG. 3 is a drawing showing an example of the relation indicated on time axes between data and an enable signal concerning the Strategy recording.
As is clear from FIGS. 1 to 3, the Strategy recording is the technique for ON/OFF controlling four current sources, which include a bottom current source 86, a read current source 87, an erase current source 88 and a write current source 83, through a controller 89, and controlling heat added to a recording layer of an optical disk by pulse-modulating the power of a laser beam output from the laser diode 85 during a recording operation onto the optical disk in order to form a proper mark on the optical disk.
Referring to the drawings, a data generator 81 is used to generate data to be recorded, a write pattern generator 82 is used to generate a write pattern, which is to be recorded as a mark on an optical disk, from the generated data, and a current controller 84 sets a current control value for the write current source 83 and supplies the set value to the write current source 83.
The enable control referred to above used for the ON/OFF controlling of the current sources, and the enable signal is a signal supplied to the LD 85 for carrying out an enable control operation.
An optical disk is generally covered with a transparent layer which prevents a recording layer from suffering any hurt or collision with a head, to maintain reading or writing. The transparent layer is formed by, for example, injection molding. Since it is difficult to accurately make the transparent layer to a desired thickness with respect to the whole surface thereof, a thickness error of several ten xcexcm occurs usually.
In order to prevent a problem by reading or writing even when the thickness error occurs, the techniques for recording information on an optical disk with a diameter of a spot thereon reduced by increasing a numerical aperture of an objective lens have heretofore been studied. In these techniques, the correcting of spherical aberration occurring due to a thickness error of a transparent layer is done. In the spherical aberration operation, for example, a degree of modulation of a reproduction signal from an optic al disk, the jitters thereof and an error rate are monitored, and a lens is driven in accordance with the content of the results of the monitoring operation.
However, when the recording of information is done on an unrecorded disk, a reproduction signal is not obtained, so that the spherical aberration cannot be compensated by a method similar to that described above. Therefore, a well-regulated mark becomes unable to be formed due to the influence of the spherical aberration to cause the recording and reproducing characteristics to be deteriorated as known well. In order to attain high-density recording on an optical disk, increasing NA (Numerical Aperture) is required as referred to above. In the case, the spherical aberration due to the influence of an error of a thickness of the transparent layer becomes a problem, so that making compensation for aberration becomes necessary.
The present invention has been made in view of the above-described problems, and provides an optical recording apparatus capable of detecting the spherical aberration, which occurs due to an error of a thickness of a transparent layer of a recording medium, even when an optical system having an objective lens of a high numerical aperture is employed, and irrespective of whether the recording medium in use is a recorded medium or an unrecorded medium; making compensation for the spherical aberration by controlling on the basis of the detected information a pulse width or amplitude of recording data with respect to the optical recording medium; and thereby lessening the deterioration due to the spherical aberration of the writing characteristics and attaining excellent recording and reproducing characteristics.
According to an aspect of the present invention, the optical recording apparatus is adapted to write information on an optical recording medium in which a recording surface is covered with a transparent layer, and comprises: means for converting recording information to be written on the optical recording medium into a pulse signal, detecting means for detecting spherical aberration occurring due to an error of a thickness of the transparent layer, and control means for controlling the pulse signal on the basis of the detected spherical aberration.
By using the above construction, when a laser recording waveform is controlled with respect to the optical recording medium by the control device on the basis of the information on the spherical aberration detected by the detecting means, the spherical aberration occurring due to an error of a thickness of the transparent layer can be corrected without obtaining a reproduction signal, and excellent recording and reproducing characteristics can be attained. Especially, the spherical aberration increases and a noticeable effect can be obtained when the NA is high, so that the present invention is effectively used for a densified optical disk.
The control means includes converting means for converting a relative value of spherical aberration obtained through the detecting means into an absolute value thereof, and generating means for generating a spherical aberration compensation value so that either one or both of the width and amplitude of the pulse signal are changed on the basis of the absolute value.
By using this arrangement, the deterioration of the writing characteristics due to spherical aberration can be reduced and excellent recording and reproducing characteristics can be realized by changing either one or both of a pulse width and amplitude of the recording data with respect to the optical recording medium on the basis of an absolute value of detected information of the spherical aberration which is outputted from the generating means through the converting means.
According to another aspect of the present invention, the optical recording apparatus is adapted to record a write pattern by pulse-modulating laser power, which is used during a recording operation, in the direction of a time axis by controlling the write current source, and the generating means controls a pulse width of the write pattern control signal by inserting a delay circuit between write pattern generating means which generates the write pattern and the write current source, the delay circuit changing the pulse width of the pulse signal by a delay period corresponding to the absolute value of the detected spherical aberration.
The generating means sets a delay period for the delay circuit to the highest level of time within a range of delay which the apparatus allows, when spherical aberration of a zero level is detected by the detecting means.
By using the arrangement, spherical aberration occurring due to an error of a thickness of a transparent layer can be corrected by changing an edge shifting value in accordance with a spherical aberration error by utilizing a phenomenon of the recording power working in the direction in which it decreases due to the influence of the spherical aberration when the spherical aberration increases, and setting long a pulse for the application of write power. This enables excellent recording and reproducing characteristics to be attained.
According to still another aspect of the present invention, the optical recording apparatus is adapted to record a write pattern by pulse-modulating laser power, which is used during a recording operation, in the direction of a time axis by controlling the write current source, and the generating means uses a variable gain amplifier which determines gain by a control voltage, for a current controller for determining a current control level of controlling a current supplied to the write current source, and controls a gain control voltage of the variable gain amplifier in accordance with the absolute value of spherical aberration to control the current control level.
By using this arrangement, a level of controlling a current supplied to the write current source is obtained by controlling a voltage for controlling the variable gain amplifier by a spherical aberration value, and this enables the spherical aberration occurring due to an error of a thickness of the transparent layer to be corrected by changing an amplitude of a pulse supplied to the write current source, and excellent recording and reproducing characteristics to be attained.
According to a further aspect of the present invention, the optical recording apparatus is adapted to record a write pattern by pulse-modulating laser power, which is used during a recording operation, in the direction of a time axis by controlling a write current source, and the generating means uses an addition circuit for a current controller for determining a current control level of controlling a current supplied to the write current source, and determines the current control level by adding an absolute value corresponding to spherical aberration to a fixed value.
By using this arrangement, a level of controlling a current supplied to the write current source is obtained by adding a value corresponding to a spherical aberration to a fixed value, and this enables the spherical aberration occurring due to an error of a thickness of the transparent layer to be corrected by changing an amplitude of a pulse supplied to a pulse current source, and excellent recording and reproducing characteristics to be attained.